Many organisms live on and disrupt devices in lakes, rivers and streams, in potable-water reservoirs, tanks, aqueducts and piping, and in industrial chemical-process plants and vats—as well as in the ocean. Our invention is beneficial for virtually every natural and artificial environment in which biofouling or other accumulations of liquid-borne materials (including nonbiological materials) disrupt or degrade equipment operations.
Thus we ask that the reader consider the following discussions as both (1) specific to the control of biofouling in seawater conductivity and temperature measurement, and (2) exemplary of the restraint of many other disruptive aggregations in liquids.
Thus the invention has broadly applicable capabilities in all these circumstances.
Both new and established observational approaches are being used for advancing fundamental understanding, monitoring, modeling, and management of the ocean environment. The ocean is tremendously complex, with countless disparate variables, each with a large range of scales. Scientific data are required at relatively high sampling frequencies and accuracies for long periods with minimal drift from calibrations. The infrastructures for such observations are developing rapidly and will include numerous dedicated oceanic observational systems. These systems will capitalize on moorings, autonomous underwater vehicles (AUVs), gliders, drifters and profiling floats.
Biofouling is a significant limitation for in-situ measurements in the coastal ocean specifically, and the whole ocean generally. Biofouling can degrade sensor accuracy and performance in a very short time, especially for contact sensors with exposed transducers and optical sensors with exposed windows. In general, for exposed windows to function adequately they must be unobstructed; this is essentially true for windows intended to transmit acoustic vibrations as well as windows intended for electromagnetic radiation. Biofouling is well known to be one of the primary limiting factors in measurement accuracy and deployment longevity for longterm oceanographic studies.
Bacteria rapidly colonize surfaces that are in contact with water. This first step of biofouling is detrimental to immersed sensing systems, as well as to industrial processes and manmade structures. The exact mechanisms of the bacterial surface colonization, particularly during its initial stages, are not fully understood. It is known, however, that once this rather fine biofilm has been established on a submerged surface, further colonization leading to biofouling by higher organisms proceeds. In advanced stages of biofouling numerous organisms degrade submerged, moored, or slowly moving sensors and measurement systems.
Attempts to solve the problem of biofouling of oceanographic sensors have used a wide variety of antifouling approaches. Various biotoxic or bioinhibiting products based on tributyl-tin (TBT) have traditionally been used to prevent fouling in longterm oceanographic instrumentation deployments. The toxic effects of TBT on mammals and the bioaccumulation of TBT in fish, oysters, and crustaceans have severely limited its longterm usefulness as a marine antifoulant. TBT-based antifoulant wax (Aquatek) and Clear-Choice aerosol spray, a polymer-based tributyl-tin methacrylate (ITW Philadelphia Resins) have been applied to areas surrounding the windows of optical instruments. Although these products are somewhat effective against algal growth, generally they are not applied directly to optical surfaces because they introduce deleterious scattering and refractive-noise errors. Ironically the roughness of these bioinhibiting coatings may provide stronger attachment for microorganisms than smooth surfaces. Additional limitations of the biodeterrent coatings include perturbed spectral transmission (window clouding) and flaking off of coatings caused by ablation. TBT compounds also have a direct negative environmental impact, as TBT is extremely toxic and should never be deliberately placed in natural waters.
Other biotoxic agents used in this field include a slowly dissolving chlorine source (trichlorisocyanuric acid); and bromine-producing tablets have been utilized in closed optical systems. These chemicals are toxic to microorganisms, preventing growth in the optical tubes. Alconox, a powdered cleaning compound (homogeneous blend of sodium dodecylbenzyl sulfonate, phosphates, and carbonates), has also been used in this manner to prohibit algal growth on optical windows.
Other such antifoulant methods for general nonoptical sensors have been used with limited success. A mixture of cayenne pepper with silicone-based grease applied to the heads of an acoustic Doppler current profiler (ADCP) helped inhibit biological growth on a bottom-mounted tripod. Upon recovery of the ADCP, extensive growth of bryozoans was noted. These animals, members of a large class of marine creatures prominent in fossils, had grown on all exposed areas of the ADCP except for the acoustic heads. Coatings of grease and pepper are not suited for optical sensors.
Zinc anodes on stainless-steel instrument cages inhibit biological growth on the stainless-steel parts. This simple and otherwise useful technique has limited application in a full antifouling approach.
Historically, copper was used extensively to protect wooden-hulled vessels from shipworms (mollusks, genus Teredo) and wood-boring crustaceans (genus Limnoria). More recently copper has been used effectively for inhibiting the growth of foulants on sensors. Copper interferes with enzymes on cell membranes and prevents cell division. As copper corrodes in seawater, copper ions are released into the water. Importantly, while copper ions are toxic at high concentrations for most organisms, they are not toxic to humans in the concentrations caused by the copper antifoulants—in contrast to TBT. Copper shutters, screens and plates have been used with some effectiveness on underwater optical sensors (Satlantic). There are numerous mechanical limitations, however, when using moving copper parts in optical systems.
Biofouling adversely affects the accuracy of electrical conductivity measurements. The contact resistances of conductivity cells change when a biofilm is deposited on its exposed surfaces. Some contact conductivity sensors use flow-through configurations, putting them at a significant disadvantage in terms of fouling. When not obstructed, these devices are known for their accuracy and stability and are standards in the industry. A conductivity sensor that uses a long flow-through tube and large electrodes to achieve its standard-setting accuracy is degraded quickly in a biofouling environment.
Some manufacturers use an inductive cell—which can be less susceptible to fouling since it has no electrodes. Such a conductivity sensor requires flow through a confined path, however, and is prone to detritus buildup and performance degradation. All such sensors could directly benefit from use of our invention to remove or prevent fouling.
Some workers in relatively remote fields have attempted to clean e.g. medical appliances by transmitted vibration; see e.g. U.S. Pat. Nos. 4,906,238 and 4,698,058. In addition to the remoteness of the fields, those proposals have been in the public eye for nearly two decades now, and are not believed to have been followed or commercially exploited.
Slightly more relevant are U.S. Pat. No. 5,384,029 of Campbell and French application 2,832,082 of Colas, assigned to Sedia, which both teach using a piezoelectric transducer for agitating or “stirring” the test fluid and cleaning the outer surface of a membrane that seals the transducer cavity—thus minimizing interference with the operation of a sensor that quantifies the presence of dissolved gas (e.g. oxygen) in wastewater or like liquid medium. Likewise relevant are European patent application 1,134,577 of Trainoff, assigned to Wyatt Technology Corporation and relating to cleaning the interior of a flow-through optical cell; U.S. Pat. No. 4,170,185 of Murphy, assigned to Lectret S. A. and directed to preventing marine fouling of a boat; and U.S. Pat. No. 5,724,186 of Collier, which concerns removing raindrops from automotive side-view mirrors.
Among several significant differences between these five references and the environments of interest to the present inventors is that none of these prior documents involves cleaning of either an active transducing surface of a sensor, or a rigid window surface for transmitting electromagnetic radiation or acoustic vibration. Other important distinctions will be seen later in this document. These twin problems of biofouling and chemical deposition have been well known for at least a half-century, but—as will also be seen—the present invention is the first to deal with either problem in a fully effective way.
To reduce the interfering effects of interfacial contact impedance, conductivity electrodes are typically platinized—that is, electrodeposited with a layer of mossy and weakly adhering microparticles of platinum. Such layers have dendritic structures, and due to their black appearance they are commonly called “platinum black.” Platinizing increases the electrode surface area greatly, thereby reducing the contact impedance of the electrode-electrolyte interface.
As can be now understood, the prior art does provide useful capabilities of measuring instruments and other equipment in immersed environments. Nevertheless the art has left important refinements to be desired in the area of longevity of measuring apparatus for best performance underwater, and likewise more generally longevity of other types of devices and other liquid environments.